1. Field of the Invention
The invention relates, generally, to a working fluid circuit for an internal combustion engine, and more specifically, to a fluid circuit for a turbocharged internal combustion engine that employs exhaust gas recirculation.
2. Description of the Related Art
Exhaust gas recirculation is commonly employed in connection with internal combustion engines as a means for controlling the generation of oxides of Nitrogen (NOx) generated during the operation of the engine. This involves the recirculation of exhaust gas byproducts, typically taken from the exhaust manifold, and routed into the intake air supply of the engine. The exhaust gas reintroduced into the engine cylinder in this way reduces the concentration of oxygen in the fuel/air mixture. A reduction of oxygen in the fuel/air mixture results in a lower maximum combustion temperature and slows the chemical reaction of the combustion process. This decreases the formation of nitrous oxides (NOx) that are discharged from the engine. In addition, the exhaust gases often contain a portion of unburned hydrocarbon that, left uncombusted, forms a part of the exhaust emissions generated during the operation of any given internal combustion engine. However, when the unburned hydrocarbons are recirculated back to the combustion chamber, they are burned thereby further reducing the emission of undesirable exhaust gas byproducts from the engine. In view of the benefits derived by employing this technique, exhaust gas recirculation is commonly found in connection with both spark ignition and compression ignition (diesel) engines. Exhaust gas recirculation is particularly useful in connection with internal combustion engines used in motor vehicles, such as passenger cars, light duty trucks, and other motorized equipment.
Turbochargers are also known to be used in the related art to provide charge air to the working fluid circuit of an engine. More specifically, when an engine is turbocharged, the pressurized exhaust gas acts on a turbine that, in turn, drives a compressor. The compressor pressurizes the intake air for the internal combustion engine making it denser. Dense intake air improves combustion resulting in increased power from the engine. Turbochargers are employed in connection with both spark ignition and compression ignition (diesel) engines for this purpose.
In addition to recirculating the exhaust gases, it is also known in the related art that lowering intake manifold temperatures reduces the formation of nitrous oxides generated as a product of combustion. However, the exhaust gases that are available for recirculation are generally very hot, sometimes exceeding 550° C. Thus, it is known in the art to cool the recirculated exhaust gas in order to lower the intake air temperature thereby further reducing the production of NOx where exhaust gas recirculation is employed. In addition, it is also known to cool the charge air delivered by the turbocharger prior to induction into the combustion chamber. The EGR intercooler and charge air cooler are separate heat exchangers that are employed to cool these two engine working fluids. One example of a turbocharged internal combustion engine having intercooled exhaust gas recirculation is found in U.S. Pat. No. 6,116,026, issued Sep. 12, 2000 and assigned to the assignee of the present invention. The disclosure of this patent is incorporated herewith.
In turbocharged internal combustion engines, the exhaust gas to be recirculated is generally removed upstream of the turbine, routed through the intercooler, and then reintroduced into the intake air stream downstream of the compressor and the charge air cooler. Exhaust gas intercoolers of this type often employ engine coolant as the cooling medium. While these coolers have generally worked for their intended purpose in the past, disadvantages still remain. More specifically, using the engine coolant as the cooling medium increases the heat load on the engine cooling system and thereby necessitates larger vehicles radiators. The use of multiple or staged coolers has also been suggested in the prior art, but this only adds to the bulk of the engine and tends to overcomplicate the engine cooling system. Furthermore, the extreme temperature differentials that exist between the exhaust gas and the coolant in the intercooler creates a harsh working environment. Some products of combustion found in the exhaust gas are highly corrosive and can condense at certain operating temperatures within the intercooler. These harsh operating environments and corrosive condensate can cause the liquid to air intercoolers to leak over time.
In response to these shortcomings, advances in the art have been made toward developing engine working fluid circuits that are capable of cooling both the recirculated exhaust gas and the charge air without the addition of multiple coolers. Additionally, these latest engine working fluid circuits have been directed at cooling the recirculated exhaust gas and charge air using an air-to-air interchanger rather than the typical liquid/air cooling interface. While these newer approaches have generally worked for their intended purposes, other drawbacks have become apparent. More specifically, the newer fluid circuits employing air-to-air exchangers must still pass the corrosive combustion by-products inherent in the EGR recirculation. While much more efficient in this process, it has been found that the EGR gases still cause a build-up of material in the transfer passages of the air-to-air exchangers, ultimately fouling the passages. These particulate materials are various types of soot and corrosive by-products of combustion that collect in the passages of the exchanger and become more troublesome when operating conditions cause condensation to form as well.
Accordingly, there remains a need in the art for an engine working fluid circuit that cools both the EGR gasses and the charge air in a single air-to air exchanger and has the ability to clean out the soot and combustion by-products that accumulate during the operation of the circuit. Additionally, there remains a need in the art for an engine working fluid circuit that has the ability to compensate for operating periods where condensation forms within the exchanger improving system reliability.